U.S. patent number 4,705,350 [Application Number 06/777,952] was granted by the patent office on 1987-11-10 for optical transmission network.
This patent grant is currently assigned to Bell Communications Research, Inc.. Invention is credited to Steven S. Cheng.
United States Patent |
4,705,350 |
Cheng |
November 10, 1987 |
**Please see images for:
( Certificate of Correction ) ** |
Optical transmission network
Abstract
An optical communications network which connects a central
office with a plurality of user stations is disclosed. In the
central office, the power from a single cw laser is divided over a
plurality of single mode optical fibers to transmit information
from the central office to the user stations. Each user station
includes a directly modulated LED for transmitting information to
the central office.
Inventors: |
Cheng; Steven S. (Bernards
Township, Somerset County, NJ) |
Assignee: |
Bell Communications Research,
Inc. (Livingston, NJ)
|
Family
ID: |
25111805 |
Appl.
No.: |
06/777,952 |
Filed: |
September 19, 1985 |
Current U.S.
Class: |
359/238;
385/24 |
Current CPC
Class: |
H04B
10/272 (20130101); H04J 14/0226 (20130101); H04J
14/0252 (20130101); H04J 14/0247 (20130101); H04J
14/0282 (20130101) |
Current International
Class: |
H04B
10/207 (20060101); H04J 14/02 (20060101); G02B
006/36 (); H04B 009/00 () |
Field of
Search: |
;350/96.15,96.16 ;370/3
;455/606,607,612 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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|
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|
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0105753 |
|
Apr 1984 |
|
EP |
|
53-41104 |
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Apr 1978 |
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JP |
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Other References
"Design & Performance of WDM Transmission System at 6.3 M
Bits", by Tetsuya et al., IEEE Transactions on Communications, vol.
Com-31, No. 9, Sep. 1983. .
Cheng et al., "Subscriber Loop Architecture", AT&T Technical
Digest, Sep. 1984, No. 75, pp. 9-10. .
Kishimoto, "Optical Coupler for Laser Redundancy System",
Electronic Letters, Jan. 4, 1982, vol. 18, p. 140. .
Gould Electronics Bulletin GD-11, Coupler Specifications--Update,
Jun. 1984, Defense Electronics Division..
|
Primary Examiner: Sikes; William L.
Assistant Examiner: Ullah; Akm E.
Attorney, Agent or Firm: Falk; James W.
Claims
I claim:
1. An optical transmission network comprising:
a central office,
a plurality of user stations, and
a plurality of bidirectional optical transmission paths for
connecting said central office with each of said user stations,
said central office comprising:
a coherent light source for emitting coherent radiation to
accommodate high-capacity, wide bandwidth data requirements,
power dividing means for dividing said coherent radiation over said
transmission paths for transmitting said coherent radiation from
said central office to said user stations,
modulating means associated with each of said transmission paths
for modulating high-capacity, wide bandwidth information onto said
coherent radiation transmitted from said central office to said
user stations, and
detector means associated with each transmission path for detecting
radiation transmitted from said user stations to said central
office; and
each of said user stations comprising,
a detector for detecting the information modulated onto said
coherent, high-capacity, wide bandwidth radiation transmitted from
said central office, and
a light emitting diode for transmitting low-capacity, narrow
bandwidth information-bearing radiation to said central office.
2. The network of claim 1 wherein each of said user stations
comprises means for directly modulating one of said light emitting
diodes.
3. The network of claim 1 wherein each of said bidirectional
optical transmission paths includes a single mode optical fiber for
transmitting said coherent radiation from said central office to
one of said user stations.
4. The network of claim 1 wherein each of said bidirectional
optical transmission paths comprises,
a single mode optical fiber for transmitting said coherent
radiation from said central office to one of said user stations,
and
a single mode optical fiber for transmitting radiation from one of
said user stations to said central office.
5. The network of claim 1 wherein at least one of said
bidirectional optical transmission paths comprises one single mode
optical fiber for transmitting said coherent radiation from said
central office to one of said user stations and for transmitting
radiation from said one user station to said central office.
6. The network of claim 5 wherein said single mode optical fiber
has a wavelength division demultiplexer/multiplex associated with
each end thereof.
7. An optical transmission network for providing communication
between a central office and a plurality of user stations, said
network comprising,
a source of coherent radiation, for transmitting high-capacity,
wide bandwidth information-carrying radiation, located in said
central office,
bidirectional optical transmissions means including a plurality of
single mode optical fibers for transmitting a portion of said
coherent radiation to each of said user stations, and
a light emitting diode located in each of said user stations for
emitting low-capacity, narrow bandwidth information-bearing
radiation to be transmitted by said bidirectional transmitting
means to said central office.
8. The network of claim 7 wherein each of said single mode optical
fibers has a modulating means associated therewith for modulating
information onto the coherent radiation transmitted to each of said
user stations.
9. The network of claim 7 wherein each of the light emitting diodes
is directly modulated.
10. The network of claim 7 wherein said single mode optical fibers
also transmit radiation from said user stations to said central
office.
11. A transmission network interconnecting a communications center
and a user station, the network comprising:
a transmission path comprising single mode optical fibers linking
the communications center and the user station,
laser means, coupled to the communications center and said path,
for transmitting coherent radiation to the user station,
modulating means associated with said path for modulating
high-capacity, wide bandwidth information onto said coherent
radiation transmitted from said central office to said user
station,
detector means associated with said path at the communications
center for detecting radiation transmitted from the user station to
the communications center,
detector means associated with said path at the user station for
detecting radiation transmitted from the communications center to
the user station, and
light emitting diode means coupled to the user station and said
path, for optically communicating low-capacity, narrow bandwidth
information from the user station to the communications center over
said path.
12. A network interconnecting a communications center with a
plurality of user stations, the network comprising:
a plurality of bi-directional, single mode optical fiber paths
interconnecting the communications center with each corresponding
one of the user stations,
the communications center further comprising:
a continuous wave coherent light source for transmitting
high-capacity, wide bandwidth coherent radiation,
power dividing means for dividing said coherent radiation among
said paths, and
wave division multiplexing means for transmitting said coherent
radiation from the communications center to each user station at a
first preselected wavelength over the corresponding one of said
paths,
each user station further comprising:
a light emitting diode for transmitting low-capacity, narrow
bandwidth information, and
wave division multiplexing means for transmitting said
low-capacity, narrow bandwidth information to the communications
center at a second preselected wavelength over the corresponding
same one of said paths.
Description
FIELD OF THE INVENTION
This invention relates to an optical transmission system, and more
particularly to a short haul optical transmission system which is
formed using low loss single mode optical fibers.
BACKGROUND OF THE INVENTION
In the first generation of single mode optical transmission
networks, low loss single mode optical fibers were deployed in
point-to-point links with directly modulated single mode lasers. A
laser is directly modulated when its drive current is directly
subjected to the modulated signal. For intercity transmissions
where long repeater spacing and high bit rate communications are of
primary concern, this is an effective and efficient use of single
mode technology.
However, for short haul communications, the typical distance
between a central office or other communications center and the
user station is much shorter than the maximum available repeater
spacing. In this environment, the use of a directly modulated laser
for each communications link between a central office and an end
user represents a considerable waste of laser power.
One way to more efficiently use single mode lasers and single mode
optical fibers in a short haul system is to share a single laser
among a plurality communications link. Laser sharing schemes for
use in single short haul optical transmission systems are disclosed
in U.S. patent application Ser. No. 680,398 filed on behalf of S.
S. Cheng J. Lipson and S. D. Personick and U.S. patent application
Ser. No. 732,556 filed on behalf of S. D. Personick. Both of these
applications are assigned to the assignee hereof and are
incorporated herein by reference.
In the systems described in the aforementioned patent applications,
bidirectional communication is established between a central office
and a plurality of user stations by means of two lasers operating
at different wavelengths and located in the central office. The
output of each laser is divided over a plurality of optical fibers
connecting the central office with the user stations so that each
fiber simultaneously transmits the two wavelengths from the central
office to the user station. The first wavelength has information
modulated onto it by means of external modulation while the second
wavelength is transmitted unmodulated. Information which is
modulated on the first wavelength is detected at the user stations
while the second wavelength is externally modulated at the user
stations and retransmitted to the central office.
Thus, the systems described in the above-mentioned patent
applications utilize single mode technology to transmit information
from the central office to the user stations and from the user
stations back to the central office. While the single mode
technology is necessary to transmit very wide bandwidth services
such as high resolution digital video from the central office to
the user stations, data transmissions from the user stations to the
central office often need only lower bandwidth capability.
Accordingly, simpler, non-single mode technology may be utilized to
provide communications between the user stations and the central
office.
It is the object of the present invention to provide a short haul
communications system which enables single mode communication from
a central office to a plurality of user stations while at the same
time providing simpler and cheaper non-single mode technology for
communications from the user stations to the central office.
SUMMARY OF THE INVENTION
The present invention is an optical transmission system for
transmitting information between a central office and a plurality
of user stations.
The central office comprises a single mode continuous wave laser
whose output is divided by a power divider over a plurality of
single mode optical fibers connecting the central office with each
of the user stations. An external modulator associated with each of
the single mode fibers is used to encode information on the
radiation transmitted from the central office to the user stations.
This information is detected by detectors located in each of the
user stations.
Illustratively, each user station also includes an LED for
transmitting information from the user station to the central
office. Typically, the LED is directly modulated as by applying the
modulating signal to its drive current. The modulated radiation
from the LED is transmitted back to the central office by means of
a single mode fiber or a multimode fiber.
In an alternative embodiment of the invention, bidirectional
wavelength division multiplexer (WDM) devices are utilized so that
only a single optical fiber is needed for each bidirectional link
between the central office and a user's station.
Thus, the optical communications system of the present invention
utilizes a shared laser over single mode fibers to transmit
information including enhanced services such as high definition
digital video from a central office to a multiplicity of user
stations, while using simpler individual LEDs with a narrower
bandwidth capability to transmit information from the user stations
to the central office.
In comparison with networks disclosed in the aforementioned patent
applications, the user stations of the present invention utilize
directly modulated LEDs instead of external modulation of radiation
transmitted from the central office. Similarly, the central office
of the present invention has only a single laser which is divided
over a set of single mode optical fibers, rather than two lasers of
different wavelengths whose outputs are divided and multiplexed for
simultaneous transmission to the user stations over the same set of
fibers.
BRIEF DESCRIPTION OF THE DRAWING
FIG. 1 schematically illustrates an optical transmission network
for connecting a central office with a plurality of user stations,
in accordance with an illustrative embodiment of the present
invention.
FIG. 2 schematically illustrates an alternative optical
transmission network for connecting a central office with a
plurality of user stations in accordance with an illustrative
embodiment of the present invention.
DETAILED DESCRIPTION
The optical transmission network of FIG. 1 comprises a central
office 101 and user stations 103-1,103-2, . . . 103-N. Information
is transmitted from the central office 101 to the user stations
103-1,103-2, . . . 103-N by means of single mode low loss optical
fibers 105-1,105-2 . . . 105-N. Information is transmitted from the
user stations 103-1,103-2, . . . 103-N to the central office 101 by
means of the single mode optical fibers 107-1,107-2, . . . 107-N
respectively. Corresponding pairs of fibers 105-1,107-1, . . .
105-N,107-N form two way optical transmission links between the
user stations 103-1 . . . 103-N. Advantageously, N may be as large
as 100.
The central office 101 comprises a single mode continuous wave
laser source 109. The laser 109 produces a wavelength in the range
of 1.3 .mu.m to 1.6 .mu.m. The laser 109 may advantageously be a
wavelength stabilized GaInAsP/InP laser such as the model
QLM-1300-Sm-BH manufactured by Lasertron Inc., Burlington, MA.
The coherent radiation produced by the laser 109 is divided over
the single mode optical fibers 105-1,105-3 . . . 105-N by means of
the 1:N power divider 111. Illustratively, the power divider 111
comprises a cascaded arrangement of one-fiber-input to
two-fiber-output couplers. An illustrative form of such a
one-fiber-input to two-fiber-output coupler is manufactured by
Gould Inc., Defense Electronics Division and is described in their
bulletin GD-11. The power divider 111 has N outputs 113-1, 113-2
and 113-N, each of which receives 1/N of the total power produced
by the laser 109.
Each of the outputs 113-1,113-2, . . . 113-N of the power divider
111 is connected to an external modulator 115-1,115-2 . . . 115-N
respectively. Such external modulators are disclosed in Trans of
IECE (Japan) Vol. E63, 1980 by M. Izutsu. Each of the external
modulators 115-1,115-2 . . . 115-N encodes information on the
coherent radiation that is transmitted from the central office 101
to the user station 103-1,103-2, . . . 103-N by means of the
optical fibers 105-1,105-2, . . . 105-N.
Each of the user stations 103-1,103-2, . . . 103-N includes a
detector 117-1,117-2, . . . 117-N respectively. The detectors
117-1,117-2, . . . 117-N detect radiation transmitted over the
fibers 105-1,105-2, . . . 105-N respectively and demodulate any
information encoded on that radiation.
Each of the user stations 103-1,103-2, . . . 103-N includes a light
emitting diode (LED) 119-1,119-2, . . . 119-N for transmitting
information back to the central office 101 via the single mode
fibers 107-1,107-2, . . . 107-N. Preferably, the LEDs 119-1 . . .
119-N are edge emitting devices. The outputs of the LEDs
119-1,119-2, . . . 119-N are modulated with information to be
transmitted to the central office by modulators 121-1,121-2, . . .
121-N. In contrast to the modulators 115-1 . . . 115-N in the
central office 101, the modulators 121-1 . . . 121-N are direct
modulators. They operate by selectively turning on and off the
current to the LEDs. Information transmitted from the user stations
103-1 . . . 103-N by way of fibers 107-1 . . . 107-N is detected in
the central office 101 by means of the detectors 123-1,123-2 . . .
123-N.
The use of a coherent single mode radiation source and single mode
low loss optical fibers enables the central office 101 to transmit
to the user stations a variety of wide bandwidth type services
including high definition digital video transmissions. However, the
return path which involves an LED and single mode fiber has a more
limited bandwidth. However, this bandwidth is suitable for most
communications between a user station and a central office. Note,
that single mode optical fibers are preferred in the return path.
However, multimode fibers may also be used.
Turning to FIG. 2 an alternative optical transmission network is
illustrated. The optical transmission network of FIG. 2 comprises a
central office 201 and user stations 203-1,203-2, . . . 203-N.
Information is transmitted from the central office 201 to the user
stations 203-1, . . . 205-N and from the user stations 203-1, . . .
203-N to the central office 201 by means of the single mode low
loss optical fibers 205-1,205-2, . . . 205-N.
Thus each of the optical fibers 205-1,205-2 . . . 205-N forms a
bidirectional optical link between the central office 101 and one
of the user stations 203-1,203-3, . . . 203-N.
Each of the fibers 205-1, . . . 205-N transmits two wavelengths of
radiation simultaneously. As shown in FIG. 2, wavelength
.lambda..sub.1, is transmitted from central office 201 to the user
stations 203-1 . . . 203-N and wavelengths .lambda..sub.2 is
transmitted from the user stations back to the central office.
Illustratively, the wavelength .lambda..sub.1 and .lambda..sub.2 in
the range of 1.3 to 1.6 .mu.m.
The wavelength .lambda..sub.1 radiation is produced in the central
office 101 by means of the single mode continuous wave laser 207.
The wavelength .lambda..sub.2 radiation is produced in the local
offices 203-1,203-2, . . . 203-N by means of LEDs 209-1,209-2, . .
. 209-N, which illustratively are edge emitting devices.
At each end of the optical fibers 105-1,105-2, . . . 105-N there is
a bidirectional wavelength division/multiplexer (WDM) device. The
bidirectional WDM devices 211-1,211-2, . . . 211-N are located in
central office 201. Similarly WDM devices 213-1,213-2, . . . 213-N
are located in the user stations 203-1,203-2, . . . 203-N
respectively. As discussed in more detail below, the bidirectional
WDM devices permit the fibers 205-1 . . . 205-N to simultaneously
transmit the wavelength .lambda..sub.1 and wavelength
.lambda..sub.2 radiation in opposite directions. Such WDM devices
are discussed in OFC'83 conference PO#1.
Returning now to the operation of the central office 101, the
wavelength .lambda..sub.1 radiation produced by the laser 207 is
divided over the fibers 205-1,205-2, . . . 205-N by means of the
1:N power divider 215 which as discussed above may be a cascaded
arrangement of a one-fiber-input to two-fiber-output couplers. The
power divider 215 has N outputs 217-1,217-2, . . . 217-N each of
which receives 1/N of the total wavelength .lambda..sub.1 power
output of the laser 207.
Each of the outputs 217-1,217-2, . . . 217-N of the power divider
is connected to an external modulator 219-1,219-2, . . . 219-N
respectively. The external modulators 219-1 . . . 219-N encode
information on the coherent wavelength .lambda..sub.1 radiation
that is transmitted from the central office 101 to the user
stations 203-1 . . . 203-N by means of the single mode optical
fibers 205-1 . . . 205-N. The modulated wavelength .lambda..sub.1
radiation exiting from the modulators 219-1 . . . 219-N is
multiplexed with the oppositely directed wavelength .lambda..sub.2
radiation transmitted over the fibers 205-1 . . . 205-N by means of
the bidirectional WDM devices 211-1 . . . 211-N.
At the user stations 203-1,203-2, . . . 203-N, the WDM devices
213-1,213-2, . . . 213-N demultiplex the incoming wavelength
.lambda..sub.1 radiation from the outgoing wavelength
.lambda..sub.2 radiation. The wavelength .lambda..sub.1 radiation
is then detected by the detectors 221-1,221-2 . . . 221-N.
In order to transmit information from the user stations
203-1,203-2, . . . 203-N to the central office 201, the LEDs
209-1,209-2, . . . 209-N are directly modulated by the modulators
223-1,223-2, . . . 223-N. The modulated wavelength .lambda..sub.2
radiation produced by the LEDs 209-1,209-2, . . . 209-N is
multiplexed with the oppositely directed wavelength .lambda..sub.1
radiation by the WDM devices 213-1,213-2, . . . 213-N. At the
central office 201, wavelength .lambda..sub.2 radiation transmitted
on the fibers 205-1,205-2, . . . 205-N is demultiplexed by the WDM
devices 211-1,211-2, . . . 211-N and is detected by the detectors
225-1,225-2, . . . 225-N.
The use of coherent radiation along with single mode low loss
optical fibers enables the central office to transmit a variety of
wide band services such as high definition digital video. Because
communication from the user stations to the central office involves
the use of an LED which emits multimode radiation onto the single
mode optical fiber, dispersion will prevent very wide band signals
from being transmitted to the central office. However, the
dispersion will not prevent more routine lower bandwidth signals
from being transmitted from the user stations to the central
office.
Thus, an optical transmission network for providing communications
between a central office and a plurality of user stations is
disclosed. Single mode technology is used to transmit information
from the central office to the user station, while simple and cheap
multimode technology is used to transmit information from the user
stations to the central office.
Finally, the above described embodiments of the invention are
intended to be illustrative only. Numerous alternative embodiments
may be devised by those skilled in the art without departing from
the spirit and scope of the following claims.
* * * * *